1. Field of the Invention
The present invention relates to an acceleration sensor, and more specifically, a semiconductor type acceleration sensor suitable for an air-bag system, a suspension control system, or the like, for automobiles.
2. Description of the Related Art
In producing a semiconductor type acceleration sensor, the movable part thereof has hitherto been prepared in such a way that it penetrates a single crystal silicon wafer. Accordingly, the movable part must be sized so as to penetrate through the thickness of a wafer, and therefore, it has been difficult to miniaturize the acceleration sensor. In addition, in order to incorporate this sensor into a package, a relaxation part, has been required, such as a pedestal, in order to release the stress caused by the difference in coefficients of thermal expansion or the like.
However, in Nikkei Electronics, Nov. 11, 1991 (No. 540), pp 223 to 231, there is illustrated an acceleration sensor produced by use of a surface micro-machining technique. According to this technique, a thin polysilicon film is laminated on a silicon substrate, and this polysilicon film is etched, whereby a beam movable parallel to the surface of the substrate is formed, so as to form a differential capacity type acceleration sensor. However, when a beam structure is formed by use of polysilicon, if a signal processing circuit is formed around the formed beam structure, the sensor characteristics become unstable. This is because the beam structure is formed of a polycrystalline and amorphous material, resulting in noticeable variation for every production lot. Accordingly, it is still desirable to form an acceleration sensor by surface micro-machining single crystal silicon.
Under such circumstances, the purpose of the present invention is to provide an acceleration sensor having a novel structure, by which high precision and high reliability can be realized.
In addition, another purpose of the present invention is to produce this acceleration sensor with a good yield rate during the production process thereof.